Ball mill grinding using cyclicly varying rotational motion



y 6, 1955 w. o. YOUNGNICKEL 2,713,976

BALL MILL GRINDING USING CYCLICLY VARYING ROTATIONAL MOTION Filed June 16, 1951 2 Sheets-Sheet l INVENTOR. WILLIAM 0. YOUNGNICKEL ATTORNEYS y 6, 1955 w. o. YOUNGNICKEL 2,713,976

BALL MILL GRINDING USING CYCLICLY VARYING ROTATIONAL MOTION Filed June 16. 1951 2 Sheets-Sheet 2 Q Q Q s E 1 Q) q M t n 4 Q R E p m )1 Q E k 2 B U k 2 i Q Q n U u E g J? 3 31:: Q Q Q!) Q 2 Q m R iwvewiow WILLIAM O. YOUNGNICKE L ATTORNEYS 2,713,976 Patented July 26, 1955 BALL MEL GRINDING USING CYCLICLY VARYKNG ROTATIONAL .MOTION Wiiliam (I1. Youngnickel, Fitchburg, Mass, assignor to Union Machine Company, :Fitchburg, Mass a corporation of Massachusetts Application June 16, 1951, Serial No. 231,933

4 Claims. Cl. 241-26) This invention relates to ball mill grinding and consists in a new and improved'process utilizing-a particular type of rotational motion and also includes within its scope a novel apparatus for carrying out this process.

The term ball mill is used herein for purposes of convenience as descriptive of all grinding apparatus wherein a vessel is rotated about a horizontal or slightly inclined axis while charged with a grinding medium and the material to be ground, and 'wherein the rotation causes the charge to roll and tumble, and usually cascade, within the vessel. Other than the ball mill proper the term includes the pebble mill, tube mill, rod mill and the like. A description of these mills and their operation will he found at page 273 et seq. of Walker, Lewis McAdams & Gilliland, Principles of Chemical Engineering (McGraW, 1937).

It is generally desirable in the operation of such mills I to rotate the mill at as high a speed aspossible in order to increase the frequency andforce of the impacts resulting from the tumbling .or cascading of the grinding medium. As the speed increases the productivityof the mill increases and the specific energy requirement de-,

creases. 'However, .the speed of rotation must be below a critical speed at which the tumbling and cascading cease and the contents of the vessel are held by centrifugal force to its walls, and rotate as a unit with the vessel. For a ball mill rotating at a uniform speed the critical speed, in R. P. M., is approximated by the formula VS-s In, Serial No. 79,615, filed March 4, 1949,, now .Patent' 2,633,303, issued March 31, .1953, described aa process of operating a ball miil wherein the angular velocity is varied several times during each revolution-thereby .mak-

ing it possible to operate at much higher speeds than have theretofore been-practical, including speeds in excess of the critical speed as the term is used .under uniform velocity conditions. In the Cumpston application the speed is varied sinusoidally.

It has now been discovered'that evengreater improvements, including increased productivity, 'lower power requirements and higher average speeds, may v.be"e" fiectively realized if the speed of rotation is cyclicly varied through a high speed pulse of short duration anda low speed dwell of long duration, the angular rotation during each period being approximately equal. A'further criterion of the speed variation pattern which characterizes this invention is a minimum .speed lower than the critical speed for uniform rotation and the maintenance of below-critical speed for an interval'sufiicient to allow or cause the charge in the mill to declerate and fall. At least one speed variation cycle should occur per revolution of the mill. Preferably, however, several cycles occur per revolution, the maximum numberbeing limited by the extent the minimum speed falls below the critical speed and the duration of the period of below-critical speed, the duration and extent neces sarily being sufficient to prevent centrifuging of the charge.

An important advantage of the speed pattern of this invention is that the minimum speed may approach the critical speed, a feature made possible by the relatively long duration of the low speed dwell. The concomitant very high speed pulse of short duration is also advan- 'tageous to milling operations in causing high acceleration and deceleration of the mill contents, thereby enhancing the impact forces and generally improving the grinding eiiiciency.

One preferred speed pattern is characterized by a maximum speed of and a minimum speed of max tiv. X

min

where X is a constant greater than 1.2. The duration of the high speed period should further be no greater than about 0.9 the duration of the low speed period, high and low being with reference to the average speed.

By operating with a maximum speed of 1.42 Wav. and a minimum speed of Wav./ 1.42, cyclicly varied eight times per revolution with a high speed period equal to 0.80 times the low speed period, an average speed 67% in excess of the uniform rotation critical speed may be maintained without causing the contents of the mill to whirl :with the vessel.

T he; preferred method of driving a ball mill according -to this characteristic speed pattern utilizes a universal .joi'nt of the type commonly referred to as a Hookes coupling (see Marks ed. lVIechanicalEngineers Handbook '(McGr-a-w-Hill, 1941), p. 940) one end of which is connectedpositivelyto the mill and the other end of which is drivenat a uniform angular velocity. With such a driving mechanism the secant of the angle between the .shafts becomes the X term in Equations 1 and 2. Thus,

an angle between'the shafts-in excess of about 30 is Whereas a cyclical variationin a cyclical variation in angular velocity of the mill will occur .2 N times (where N represents the gear reduction ratio) per revolution.

Aconstant speed drive of the ,power input endof the universal joint is conveniently provided by keying 'the input 'shaft to a flywheel having a sufficient momentof Zinert-ia'.(depending of course on thespeed and Weight of the mill .and the magnitude of the velocity variations) 'to-maintain thetdesired constant angular velocity. Other equivalent :rn'ethodsof maintaining asubstantially conustant angular velocity may also be used.

.One modification of theapparatus of this invention .provides'a variable shaft angle at the universal joint, :whereby the magnitude of the velocity variations may be varied. Conveniently the driving unit or the mill unit or bothare pivoted on bushings which are axially aligned with the center of the joint.

Where larger speed variations than can be provided by a single universal joint are desired, a pair of universal of the couplings.

joints, connected by an intermediate shaft, may be provided, and arranged so that their effectiveness in causing variations in angular velocity is additive. The maximum effectiveness is attained when the planes in which lie the forks of the universal joints at the ends of the intermediate member are at right angles. With this arrangement the factor X in Equations 1 and 2 becomes the product of the secants of the two shaft angles.

Accordingly this invention includes not only a method of driving a ball mill for greater productivity and lower power requirements, but also an apparatus in which the desired driving conditions may be realized and optionally varied as desired. It will be seen that the apparatus is simple in its construction as well as rugged and easy to maintain.

This invention will be best understood from the following detailed description of a preferred embodiment thereof, selected for purposes of illustration, and from the accompanying drawings in which:

Fig. l is a plan view of a preferred apparatus having a pair of universal joints additively arranged, with each contributing to produce the desired speed variation pattern,

Fig. 2 is a front elevation of the apparatus shown in Fig. l, but with the motor unit axially aligned with the intermediate coupling, and r Fig. 3 is a graph showing one preferred speed variation pattern.

The mill comprises an octagonal drum 1 closed at one end by a plate 2 which projects beyond the sides of the drum thereby presenting a flange, and closed at the other end by a cover 3 secured by bolts. The mill is mounted on a face plate 4 by means of clamps 5 which engage the flange'of the plate 2.

As herein shown the mill is driven through a pair of universal joints (Hookes couplings) 6 and 7, connected by an intermediate shaft 8, and through a gear reduction train 9 to the output shaft 10 of which is secured the face plate 4. The yokes of the universal joints 6 and 7 on the intermediate shaft are at right angles. The mating yokes are connected respectively to the input shaft 11 of gear train 9 and to power shaft 12 to which is also keyed a flywheel 13. The power input shaft 12 is driven by any appropriate source of power, conveniently an electric motor 14 and a variable speed controller 26.

The entire assembly will be seen to comprise two units, the power unit and the mill unit connected through the pair of Hookes couplings. Each unit is mounted on a sub-base 15 and 16 which may be rotated in a horizontal plane about axes which pass through the center points vided in the sub-base members, having axes aligned with the centers of the couplings, engage corresponding journals 19 and 20 of a supporting frame 21. Two series of tapped holes 22 and 23 arcuately spaced about the journals are conveniently provided, along with a hole 24 and 25 in each base member, whereby each base member may be bolted in its adjusted position. Preferably the tapped holes are arranged so that the base members may be fixedly secured in positions angularly displaced from the linear by equal amounts, so that the angle between the power shaft 12 and the intermediate shaft 8 equals the angle between the input shaft 11 and the intermediate shaft 8, and so that the two shafts are displaced in opposite rotational directions.

In operation the angles between the power shaft and intermediate shaft and between the input shaft and intermediate shaft are set to give the desired magnitude of speed variation by swinging the base members on their pivots and bolting them in position. The mill is then charged with the desired grinding medium, e. g. steel balls, and material to be ground, covered and secured to the face plate. The apparatus may then be placed in operation, using the variable speed drive to control the average speed of rotation.

As shown, bushings 17 and 18 pro- 4 In a typical run under the following operating conditions Mill:

Formoctagonal Diameter, 8" Length, 5 /2" Load, 18# steel balls /2"; 4 /2 lbs. sand-1O mesh Speed, average 150 R. P. M.; critical (for uniform speed) R. P. M. Gear reduction, 4:1 Universal joint settingseach at 45 Flywheel, 90# rim weight the rate of production of product of a given size was about 4 times that produced under conventional operating conditions of 60% critical speed and the power requirements were reduced by from 50 to 60 per cent.

The speed pattern of this mill under the above described operating conditions is shown in Fig. 3. From there it will be seen that during the high speed period A, of relatively short duration, the acceleration and deceleration are high as represented by the slope of the curve. It is noteworthy that only during a fraction B of the low speed dwell C is the speed below the critical speed, and then only slightly below it. This effect is believed attributable to the relatively long low speed period wherein the deceleration and acceleration forces are relatively low, so that the charge falls very shortly after the speed becomes less than critical. Because of this rapid fall, unusually high average speeds may be maintained.

The preferred mill described above comprises an octagonal vessel but other forms may also be used. It has been found that polygonal shaped vessels are advantageous; the intersection of the planes comprising the walls appear to provide better lifting of the contents as well as deflectors which help to cause the contents to decelerate and fall during the low speed periods.

Other obvious modifications of the preferred apparatus will also be apparent, such as the provision of a fixed angular relationship between the drive unit and mill unit, an increase or decrease in the number of universal joints and other methods of varying the angular relationship between the drive unit and mill unit. Furthermore, it should be apparent that the improved speed pattern of this invention may be attained by mechanisms other than universal joints. These and other modifications are considered as being within the scope of this invention.

Having thus disclosed my invention and described in detail a preferred embodiment thereof, I claim and desire to secure by Letters Patent:

1. In the process of grinding by means of a ball mill or the like that improvement which comprises rotating the mill including its charge about a substantially horizontal axis at a cyclicly varying angular velocity the average of which is greater than the critical speed, at least one cycle occurring per revolution of the mill, each cycle including a high speed pulse and a low speed dwell, the maximum speed being at least 1.2 times the average speed and the minimum speed being no higher than 1/ 1.2 times the average speed, the period during which the speed is higher than average being less than 0.9 times the period during which the speed is lower than average, there being included during each low speed dwell a period during which the speed is below the critical speed and of sufficient duration to prevent centrifuging of the charge.

2. A ball mill or the like comprising a polygonal drum mounted for rotation about a horizontal axis, and a driving mechanism for said drum including a pair of angu larly disposed shafts connected together by a pair of Hookes couplings joined by an intermediate shaft, the coupling members at the ends of said intermediate shaft lying in perpendicular planes, one of said shafts being positively connected through gear reduction means to said drum and means for driving the other of said shafts 5 at a uniform angular velocity, the angles between each of said shafts and said intermediate shaft being such that the product of their secants is in excess of 1.2.

3. A ball mill having a driving shaft in combination with a motor having a driven shaft, a universal joint on each shaft, an intermediate shaft connecting said universal joints, a base, mill supporting means for connecting the mill to the base for pivotal movement about the axis of one of said joints, motor supporting means for connecting the motor to the base for pivotal movement about the axis of the other of said joints, means for securing the mill supporting means to the base in different positions of angular displacement, and means for securing the motor supporting means to the base in difierent positions of angular displacement.

4. A ball mill or the like comprising a drum mounted for rotation about a horizontal axis, a driving mechanism for said drum including a pair of angular-1y disposed shafts connected together by a pair of Hookes couplings joined by an intermediate shaft, the coupling members at the :3,

ends of said intermediate shaft lying in perpendicular planes, one of said shafts being positively connected to said drum and means for driving the other of said shafts at a substantially uniform angular velocity, the angles between each of said shafts and said intermediate shaft 6? being such that the product of their secants is in excess of 1.2.

References Cited in the file of this patent UNITED STATES PATENTS 419,241 Bitley Ian. 14, 668,873 Dickinson Feb. 26, 1901 686,027 Carpenter Nov. 5, 1901 706,102 Pendleton Aug. 5, 1902 908,861 Hardinge Jan. 5, 1909 1,069,705 Kluge Aug. 12, 1913 1,225,061 Schultz May 8, 1917 1,328,872 Harris Jan. 27, 1920 1,468,893 Wolever Sept. 25, 1923 1,899,170 Wainwright Feb. 28, 1933 1,933,764 Ruth, Jr Nov. 7, 1933 1,948,328 Boughton Feb. 20, 1934 2,469,484 Thiman May 10, 1949 FOREIGN PATENTS 499,556 Germany June 7, 1930 526,921 France July 5, 1921 637,123 France Jan. 24, 1928 

